1. Field of the Invention. This invention relates broadly to the field of well-logging, and more particularly to the field of measuring the course length of a drilled well bore. Specifically, this invention relates to that class of instruments and methods for measuring course length that employs a probe containing accelerometers that yield a signal indicative of the time rate of change of velocity of the probe as it is run through the well bore. This signal is then processed to yield a signal representing the distance traveled by the probe.
2. Brief Description of the Prior Art. Accurate course length measurement is a basic requirement for the accurate survey of a well bore, assuring, for example, entry of the bore into the desired geologic formation. The course length of a well bore is the distance from the surface to the bottom of the bore along the bore's length. If the well bore is entirely vertical, the course length is essentially the same as the vertical depth. In many, if not most, cases, however, a substantial portion of the well bore is drilled at an angle from true vertical, a technique known as "slant" or "directional" drilling. In such cases, the course length of the bore will be greater than its vertical depth.
A conventional technique for measuring course length uses a cable that is run through the bore. A wheel is turned by the cable as it is lowered, and the rotations of the wheel are counted. The distance traveled by the cable down the bore is calculated by multiplying the number of rotations of the wheel by its known circumference. This method yields a low degree of accuracy, due to slippage between the cable and the wheel, and stretching of the cable as a result of its own weight.
Limitations inherent in the cable method led to the development of well bore probes that employ an accelerometer that produces a signal which is integrated twice to yield a value indicative of distance. One accelerometric method, for example, employs an accelerometer that is maintained in a vertical orientation, by gyrometric means, as the probe is lowered. Another method employs three accelerometers oriented along three orthogonal axes.
In U.S. Pat. No. 4,545,242 to Chan, for example, a basic course length value is derived from cable length sensing means, with an accelerometrically-derived signal used as a correction factor.
With these accelerometric methods (and their several variations), it is necessary to correct for gravitational effects. Gravitational forces generate perturbation effects on the acceleration measurement along all three axes due to the inclination of the bore, and due to the effects of the geologic formations between the bore and the surface. Thus, the magnitude of the gravitational effect on the acceleration measurement may change as a function of depth and geologic formation, and the direction of the effect on an accelerometer may change as a function of bore inclination. By maintaining the accelerometer vertical, or by using three orthogonally-oriented accelerometers, these gravitational factors can be offset to a significant extent.
Nevertheless, additional sources of error in the acceleration measurement exist which can diminish the accuracy of the distance measurement. For example, since the magnitude of the gravitational forces on the accelerometric means changes with depth, the accelerometer probe must be stopped periodically for recalibration of its "zero" reading. In other words, the correction factor for the force of gravity must be periodically updated as the magnitude of that factor changes, so that a true reading of dv/dt is obtained. This necessitates bringing the probe to an absolute stop, since any residual motion can impart an erroneous "zero" reading. Not only is stopping the probe time consuming, it is also a tricky feat, mechanically. Yet, the need to obtain an accurate "zero" is great, since error in the accelerometer signal can result in accumulated error in the distance measurement.
Thus, there is a need, as yet unsatisfied, to achieve accurate accelerometric measurement of course length without the need to stop the probe periodically for recalibration. In other words, "real time" recalibration is desired. Furthermore, enhanced overall accuracy, as compared to current techniques, is also sought.